The present invention relates to a semiconductor light-emitting device in a triode configuration such as a light-emitting diode device or a semiconductor laser device and to an apparatus for driving the same.
Light-emitting diode devices have been used widely as low-cost and high-reliability light-emitting devices in remote control equipment and optical fiber communication.
However, conventional light-emitting diode devices have the problems of low response speed and low upper-limit modulation frequency in performing high-speed communication, i.e., high-speed modulation.
Factors that limit the operating speed of a semiconductor light-emitting device represented by a light-emitting diode device include the speed at which carriers injected in the active layer are recombined. The carriers injected in the active region of the light-emitting device do not disappear immediately after current injection is halted but disappear gradually in accordance with a time constant determined by the recombination speed.
Since the light-emitting state continues while the carriers remain in the active region, the carriers remaining in the active region prevent high-speed response of the light-emitting device during modulation. Since the light-emitting diode device utilizes spontaneous light emission and the amount of light emitted therefrom is nearly proportional to the quantity of carriers in the active region, the remaining carriers exert particularly great influence on the response speed of the light-emitting diode device. In a light-emitting diode device composed of a Group III-V compound semiconductor containing, e.g., aluminium gallium arsenide (AlGaAs) as a main component, the time constant determined by the carrier recombination speed is normally several nanoseconds (ns) so that it is difficult to perform high-speed modulation at a modulation frequency exceeding 1 GHz.
As prior art technology for eliminating the limit placed by the carrier recombination speed on the modulation speed, a light-emitting device using a triode configuration similar to that of a transistor device is disclosed in Japanese Unexamined Patent Publication No. SHO 60-167390.
FIG. 17 shows a cross-sectional structure of the triode light-emitting device disclosed in the publication.
As shown in FIG. 17, the semiconductor light-emitting device disclosed in the publication comprises a p-type collector layer 902, an n-type base layer 903, and a p-type emitter layer 905 formed successively on a p-type semiconductor substrate 901, similarly to a bipolar transistor.
An active layer 904 is provided between the base layer 903 and the emitter layer 905. The active layer 904 is surrounded by an n-type buried layer 907 formed in the peripheral region thereof.
An emitter electrode 909 is formed on the emitter layer 905 with a p-type contact layer 906 interposed therebetween. A base electrode 910 is formed on the buried layer 907 with an n-type contact layer 908 interposed therebetween so as to surround the emitter electrode 909. A collector electrode 911 is formed on the surface of the semiconductor substrate 901 opposite to the collector layer 902.
A description will be given herein below to the operation of the conventional semiconductor light-emitting device.
FIG. 18 shows the structure of electron energy bands in the conventional semiconductor light-emitting device during a light-emitting period, in which the vertical axis represents the energy of electrons and EC, EV, and EF generally represent energy at the lower end of the conduction band, energy at the upper end of the valence band, and the energy of electrons or holes on a quasi-Fermi level, respectively. The reference numerals associated with the energy levels correspond to the semiconductor layers shown in FIG. 17.
As an example of driving voltage applied during the light-emitting period, a voltage in a forward direction (forward bias voltage) is applied between the base layer 903 and the emitter layer 905 such that the base layer 903 and the collector layer 902 are set at an equal potential of 0 V.
Since the forward bias voltage is applied between the base layer 903 and the emitter layer 905, electrons injected from the base layer 903 and holes injected from the emitter layer 905 are accumulated in the active layer 904 and recombined to emit light. Although a depletion layer is formed between the p-type collector layer 902 and the n-type base layer 903 due to the pn junction, at least a part of the base layer 903 is not depleted so that the electrons are supplied from the undepleted portion to the active layer 904. The base layer 903 functions as a barrier for confining the holes to the active layer.
During a light-extinct period, a voltage in a reverse direction (reverse bias voltage) is applied between the base layer 903 and the collector layer 902. This depletes substantially the entire region of the base layer 903, as shown in the energy-band diagram of FIG. 19, so that the holes confined to the active layer 904 are extracted to the collector layer 902. If the holes can be extracted from the active layer 904 with sufficiently high efficiency, the concentration of the holes in the active layer 904 is reduced so that the quantity of carriers recombined for light emission is reduced and light emission is suppressed. Since the hole extracted operation is not dependent on the speed carrier recombination for light emission, light emission can be halted promptly so that high-speed modulation is allowed.
As a result of conducting various studies on the conventional semiconductor light-emitting device in the triode configuration, the present inventors have found the problem that, if low-voltage driving is performed during a light-extinct operation, some of the holes remain in the active layer 904 and emitted light remains even during the extinction period. Briefly, it is difficult to achieve a high extinction ratio, which is the ratio between the amount of light during the light-emitting period and the amount of light during the extinction period.
FIG. 20 shows in enlarged relation a band structure at the upper end of the valence band in the active layer 904 and its vicinity in the conventional semiconductor light-emitting device during the extinction period. As shown in FIG. 20, an interface barrier (spike) 920 occurs between the active layer 904 and the base layer 903 during the extinction period due to the offsetting of the valence band caused by the heterojunction therebetween. Even if the absolute value of the potential of the reverse bias voltage applied to the collector layer 902 is increased, the height of the interface barrier 902 (the magnitude of energy) does not change, which forms an obstacle to the extraction of the holes to the collector layer 902. Although some of the holes move toward the collector by surpassing the interface barrier 902 with the reverse bias voltage, holes with energy lower than the height of the interface barrier 902 remain at the interface between the active layer 904 and the base layer 903. If a higher reverse bias voltage is applied, some of the holes with lower energy are transported by a tunnel current to the collector layer 902 but the reverse bias voltage with the higher absolute value also increases the amount of heat generated from the device as well as power consumption.
At this time, the holes are supplied from the emitter layer 905 to the active layer 904 so that, if the concentration of the holes is increased at the interface between the active layer 904 and the base layer 903, the quantity of holes accumulated in the entire active layer 904 is increased. In the conventional semiconductor light-emitting device, therefore, it is difficult to sufficiently reduce the quantity of holes in the active layer 904 with a low reverse bias voltage and a considerable amount of light is emitted from the active layer 904 even during the extinction period.
Thus, it is difficult to achieve a higher extinction ratio in the conventional semiconductor light-emitting device in the triode configuration during the low-voltage driving.
It is therefore an object of the present invention to allow high-speed operation with a low voltage and provide a practical extinction ratio by solving the conventional problems.
To attain the foregoing object, a first semiconductor light-emitting device according to the present invention comprises: first and second semiconductor layers each of a first conductivity type; a third semiconductor layer of a second conductivity type provided between the first and second semiconductor layers; an active layer provided between the second and third semiconductor, the active layer emitting light with charge injected therein from the second and third semiconductor layers; and a graded composition layer provided between the active layer and the third semiconductor layer to have a varying composition which is equal to a composition of the active layer at an interface with the active layer and to a composition of the third semiconductor layer at an interface with the third semiconductor layer.
If the third semiconductor layer of the known semiconductor light-emitting device is a base layer, the active layer and the base layer are composed of heterojunctions, as described above. Accordingly, a band offset causes an interface barrier when a reverse bias voltage is applied during an extinction period. However, the first semiconductor light-emitting device of the present invention has the graded composition layer between the active layer and the third semiconductor layer, which eliminates the band offset and therefore prevents the occurrence of the interface barrier. As a result, even a low reverse bias voltage achieves a sufficient reduction in the quantity of carriers remaining in the active layer so that a higher extinction ratio is achieved by low-voltage driving.
A second semiconductor light-emitting device according to the present invention comprises: first and second semiconductor layers each of a first conductivity type; a third semiconductor layer of a second conductivity type provided between the first and second semiconductor layers, the third semiconductor layer having a forbidden band as an electron energy band which is smaller in width than a forbidden band in each of the first and second semiconductor layers; and a graded composition layer provided between the first and third semiconductor layers to have a varying composition which is nearly equal to a composition of the first semiconductor layer at an interface with the first semiconductor layer and to a composition of the third semiconductor layer at an interface with the third semiconductor layer, the third semiconductor layer emitting light with charge injected therein from the second and third semiconductor layers.
If the third semiconductor layer of the second semiconductor light-emitting device is the base layer, the base layer functions as a substantial active layer since the forbidden band width in the base layer is smaller than the forbidden band width in each of the first and second semiconductor layers. Thus, even in the semiconductor light-emitting device which does not have an independent active layer, the graded composition layer provided between the first semiconductor layer (collector layer) and the third semiconductor layer (base layer) eliminates the band offset and therefore prevents the occurrence of the interface barrier. As a result, even a low reverse bias voltage achieves a sufficient reduction in the quantity of carriers remaining in the third semiconductor layer so that a higher extinction ratio is achieved by low-voltage driving.
A third semiconductor light-emitting device according to the present invention comprises: first and second semiconductor layers each of a p-type conductivity; and a third semiconductor layer of an n-type conductivity provided between the first and second semiconductor layers, the third semiconductor layer having a forbidden band as an electron energy band which is smaller in width than a forbidden band in each of the first and second semiconductor layers, the third semiconductor layer emitting light with charge injected therein from the second and third semiconductor layers, an energy value at an upper end of a valence band as an electron energy band being lower in the first semiconductor layer than in the second semiconductor layer.
In the third semiconductor light-emitting device, the third semiconductor layer functions as a substantial active layer, similarly to the second semiconductor light-emitting device of the present invention. If the first semiconductor layer is a collector layer and the second semiconductor layer is an emitter layer, an energy value at the upper end of the valence band is lower in the collector layer as the first semiconductor layer than in the emitter layer as the second semiconductor layer. This suppresses current injection from the collector layer without interrupting current injection from the emitter during a light-emitting period. This also suppresses a leakage current from the emitter layer to the collector layer and achieves a higher extinction ratio.
A fourth semiconductor light-emitting device according to the present invention comprises: first and second semiconductor layers each of an n-type conductivity; and a third semiconductor layer of a p-type conductivity provided between the first and second semiconductor layers, the third semiconductor layer having a forbidden band as an electron energy band which is smaller in width than a forbidden band in each of the first and second semiconductor layers, the third semiconductor layer emitting light with charge injected therein from the second and third semiconductor layers, an energy value at a lower end of a conduction band as an electron energy band being higher in the first semiconductor layer than in the second semiconductor layer.
In the fourth semiconductor light-emitting device, the third semiconductor layer functions as a substantial active layer, similarly to the second semiconductor light-emitting device of the present invention. If the first semiconductor layer is a collector layer and the second semiconductor layer is an emitter layer, an energy value at the lower end of the conduction band as an electron energy band is higher in the collector layer as the first semiconductor layer than in the emitter layer as the second semiconductor layer. This suppresses current injection from the collector layer without interrupting current injection from the emitter during the light-emitting period. This also suppresses a leakage current from the emitter layer to the collector layer and achieves a higher extinction ratio.
In each of the second to fourth semiconductor light-emitting devices, an impurity concentration in the second semiconductor layer is preferably higher at least in a region thereof opposed to the first semiconductor layer than in the first semiconductor layer. If the first semiconductor layer is a collector layer and the second semiconductor layer is an emitter layer, the second semiconductor layer is higher in impurity concentration than in the first semiconductor layer so that the efficiency of carrier injection from the second semiconductor layer (emitter layer) is improved.
A fifth semiconductor light-emitting device according to the present invention comprises: first and second semiconductor layers each of a first conductivity type; a third semiconductor layer of a second conductivity type provided between the first and second semiconductor layers, the third semiconductor layer having a forbidden band as an electron energy band which is smaller in width than a forbidden band in each of the first and second semiconductor layers; and a lightly doped semiconductor layer provided between the first and third semiconductor layers, the lightly doped semiconductor layer having an impurity concentration which is lower than an impurity concentration in each of the first and third semiconductor layers, the third semiconductor layer emitting light with charge injected therein from the second and third semiconductor layers.
In the fifth semiconductor light-emitting device, the third semiconductor layer functions as a substantial active layer, similarly to the second semiconductor light-emitting device of the present invention. If the first semiconductor layer is a collector layer, the potential gradient in the interface barrier between the third semiconductor layer (base layer) and the first semiconductor layer (collector layer) becomes sharp during the extinction period due to the lightly doped semiconductor layer provided between the first and third semiconductor layers, which prevents carriers from remaining in the interface barrier portion. As a result, even a low reverse bias voltage achieves a sufficient reduction in the quantity of carriers remaining in the third semiconductor layer so that a higher extinction ratio is achieved by low-voltage driving.
In the fifth semiconductor light-emitting device, the lightly doped semiconductor layer is preferably an undoped layer undoped with an impurity.
In the fifth semiconductor light-emitting device, the lightly doped semiconductor layer preferably has the second conductivity type. In the arrangement, the lightly doped semiconductor layer provided between the first semiconductor layer (collector layer) and the third semiconductor layer (base layer) forms a pn junction between itself and the first semiconductor layer. During the light-emitting period, therefore, a barrier against carriers injected from the first semiconductor layer (collector layer) to the third semiconductor layer (base layer) occurs during the light-emitting period even with the application of a forward bias voltage between the collector and the base. The barrier prevents carrier injection in a reverse direction from the first semiconductor layer (collector layer) even if the first semiconductor layer (collector layer) and the second semiconductor layer (emitter layer) are set at equal values.
An apparatus for driving a semiconductor light-emitting device according to the present invention assumes an apparatus for driving a semiconductor light-emitting device comprising first and second semiconductor layers each of a first conductivity type and a third semiconductor layer of a second conductivity type provided between the first and second semiconductor layers, the apparatus comprising: constant-current control means; light-emission control means for controlling a state of light emitted from the semiconductor light-emitting device; and specified-potential applying means for applying a specified potential to the third semiconductor layer of the semiconductor light-emitting device, the constant-current control means supplying a specified driving current to the second semiconductor layer of the semiconductor light-emitting device, the light-emission control means adjusting an amount of light emitted from the semiconductor light-emitting device by applying different voltages to the first semiconductor layer or by bringing the first semiconductor layer into different states of impedance.
The apparatus for driving a semiconductor light-emitting device according to the present invention ensures the light-emitting and light-extinct operations of a semiconductor light-emitting device in a triode configuration.